Follow-up refactoring of Payload

[philippecamacho]

The Payload struct has been redesigned (see #1012) however there are some pending improvements including:

• Get rid of type TxTableEntry See

  espresso-sequencer/sequencer/src/block2/entry.rs

  Line 10 in dbf68c9

  pub type TxTableEntryWord = u32;

• This type should be replaced with TableWord with the appropriate trait bounds which will likely involve the use of const generics.
  • Don't allow arithmetic directly on table entries: zero(), one(), checked_add_mut()
• Change BlockPayload::Encode to avoid copying the payload.
• Add unit tests for NameSpaceTable
• Add unit tests for TxTable
• Handle errors
• See if the TxTable / NameSpaceTable can store only a reference to the payload